The computation of a navigation solution for an object at rest or in motion based on measurements of the changes in the inertia of the object is a well-established science. Instruments known as Inertial Measurement Units (IMU's) comprising sensors and computational functions have been developed for a wide variety of navigation tasks such as charting the course of a submarine below the surface of the water or guiding the trajectory of a missile in space. The task of an IMU is to measure the changes in acceleration experienced by the object from which a navigation solution or update can be calculated. Typically, the type of sensors that are used in an IMU are either a gyroscope or accelerometer, or both. The number of sensors depends upon the number of independent measurements that are desired for the specific computational process employed and the accuracy of the solution desired. The resulting accuracy of the navigation solution is highly dependent upon the sensitivity and accuracy of the sensors, the resolution of the data they provide to the computational process, and the speed and resolution of the computations themselves. The cost of an IMU for typical navigation purposes can range from a few thousand dollars to over a million dollars depending on the accuracy and other requirements of the navigation function.
Today, there are some low cost accelerometers have been produced for commercial markets that will measure a tilt which is due to changes in the direction of gravity. One example is a tilt sensor that can be built into a device to provide an automatic shut-off capability in case the device is tipped on its side. Another application of low accelerometers is in the automotive industry where an automobile's sudden deceleration can be detected and used to trigger the deployment of an air bag protective device during a collision. These low cost accelerometer sensors have been designed for applications that do not require a high degree of measurement accuracy as compared to accelerometers designed for navigation applications.
It would be desirable for certain commercial applications to employ both low cost sensors and low cost digitization and computational elements in order to be able to include inertial navigation capabilities.
Low cost navigation solutions based upon inexpensive sensors and computational elements are available. Incorporation of accelerometers, for example, is being incorporated into hand held computer displays. Examples of this type of display can be found, for example, in U.S. patent application Ser. No. 09/328,053, filed Jun. 8, 1999, which is incorporated herein by reference. In the '053 application, a hand held device displays a portion of a computer image, and a user scrolls this image by moving the hand held device in real space.
What is needed, therefore, is a way of making a viable inertial navigation capability using low cost accelerometers, while at the same time maintaining a practical level of navigation accuracy, complexity and cost.